Reductive Decolorization of Azo Dye by Bacteria - Microbial Degradation of Synthetic Dyes in Wastewaters - page 109

uents into natural water bodies
undoubtedly causes aesthetical unpleasantness. In addition, it leads to a reduction in
sunlight penetration, which, in turn, decreases photosynthetic activity and dissolved
oxygen amount in the water, and affects aquatic

The improper discharge of colored dye ef

ora and fauna (Saratale et al.
2011 ). However, due to increasing stringent government legislation against dye
pollution all over the world, serious attention has been paid for the treatment of dye
wastewater. We would like to focus here on the bacterial decolorization of azo dyes.
Recent studies on factors affecting the decolorization performance, the involvement
of azoreductase in decolorization, potential mechanisms of azo dye decolorization,
and toxicity evaluation of azo dyes and their decolorized metabolites are presented
here.

2 Bacterial Decolorization of Wastewater Containing Azo
Dye

Based on their chemical structures and chromophores, synthetic dyes could be
classi 10 3 azo dyes, containing

×

ed into 20 - 30 different groups. More than 2 - 3

one or more groups, constitute the largest commercially available class of

synthetic dyes (Stolz 2001 ; Pandey et al. 2007 ; Vijaykumar et al. 2007 ; Saratale
et al. 2011 ). It was estimated that azo dyes make up around 70 % of all dyestuffs
used worldwide by weight (Zollinger 1987 ). Azo dyes are easy to be synthesized,
encompass all colors, attach well to fabrics and have good fastness properties
(Chengalroyen and Dabbs 2013 ). Because of their high chemical, biological and
photocatalytic stability and resistance to breakdown caused by time, water and
soap, microbes, and sunlight exposure, azo dyes are widely used in many different
industries including textile dyeing, paper printing, leather, photography, food,
pharmaceuticals and cosmetics etc. (Pandey et al. 2007 ; Sol

- N=N -

s et al. 2012 ). Due to
the wide use and release of azo dye into the environment, many studies on the
treatment of azo dye wastewater have been carried out during the past decades.
Physicochemical methods, like adsorption, coagulation/

í

occulation, advanced

oxidation, and membrane ltration etc., have generally inherent drawbacks

including intensive energy and cost input, generation of large amounts of sludge
that may cause secondary pollution, and complicate operation procedures (Saratale
et al. 2011 ). Therefore, more environment-friendly and cost-competitive alterna-
tives are needed. To date, only one naturally occurring azo compound (4, 4

-
dihydroxy azo benzene) has been reported (Gill and Strauch 1984 ). Thus, all the
other azo compounds are xenobiotic and expected to be recalcitrant to biodegra-
dation. It was observed that azo dyes generally resist biodegradation in conven-
tional activated sludge treatment units (Stolz 2001 ). The adaptability and activity of
selected microbes determine their effectiveness in microbial decolorization.

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